1. Field of the Invention
This invention relates to a liquid crystal display device, and in particular, to a field-effect liquid crystal display device and a phase plate therefor.
2. Description of the Related Art
A conventional twisted nematic type liquid crystal display element has a 90 degrees-twisted helical structure of a nematic liquid crystal having positive dielectric constant anisotropy between two electrode substrates, and a polarizer is disposed outside each of the above electrode substrates such that its polarization axis is at right angles or parallel with liquid crystal molecules adjacent to the electrode substrate.
Such a liquid crystal display element having a twist angle of 90 degrees has problems concerning steepness between voltage charged to a liquid crystal layer and transmissivity of the liquid crystal layer, and visual angle characteristic, and hence, achievement of high duty multiplexing is practically limited.
Further, in order to meet demands for display quality improvement and information capacity increase in a liquid crystal display element, a device using a display element of super twist birefringence type (STN) where the liquid crystal between two electrodes is oriented by twisting it at 180 degrees or more has been put to practical use. However, this device has a problem that it can only achieve display of limited colors, e.g. display of blue in yellow ground or display of blue in white ground.
In recent years, a display element of STN type free from coloring unlike the above has been desired, and there has therefore been proposed a method of forming a double layer structure of liquid crystal elements by stacking them.
That is, there is a proposal for a complete black and white display formed by stacking display elements having an identical twist angle and .DELTA.n.d and a different rotation angle (Okumura, Nagata and Wada: Television Society Technical Report, Vol. 11, No. 27, page 79, 1987).
However, when the STN liquid crystal elements are used to form the double layer structure, the margin of .DELTA.n.d is small, and it is difficult to control the cell gap. And, two pairs of the STN liquid crystal elements are required, or increase the cost. Therefore, the mass producibility of such a device is low.
In particular, in large-sized display devices which are used in office automation appliances and require two pairs of glass plates as cell substrates, the above double layer structure is undesirable, since it is a large factor to prevent the weight decrease thereof.
For this reason, there is another proposal for black and white display by phase compensation using, in combination, one liquid crystal element and a phase plate of a plastic film having birefringence characteristic (Nagae, Hirakata and Komura: Television Society Technical Report, Vol. 12, No. 32 page 29, 1988).
The above display device using a plastic film phase plate in combination has a lower contrast than the display device using two pairs of liquid crystal display elements. In particular, an adhesive necessary for attaching the film has an influence, and the contrast therefore decreases according to the number of the attached films.